1. Field of the Invention
The invention relates to a process for removing plastics from microstructures, in particular those microstructures having a large aspect ratio. Structures of this type have structure depths ranging from a few microns to millimeters, with lateral dimensions in the micron range.
2. Discussion of the Background
Microstructures can be produced by various processes, for example by means of the LIGA process, or by micromechanical precision machining. The LIGA process comprises the process steps of lithography, electroforming and casting; it can be used for the precision manufacture of large numbers of microstructures.
The preferred lithography production method is X-ray depth lithography using a radiation-sensitive resist, such as polymethyl methacrylate (PMMA), which is preferably crosslinked. Compared with uncrosslinked PMMA, crosslinked PMMA adheres better to the substrate, is more shape-stable and has more favorable mechanical properties and higher selectivity during development.
During electroforming, a metal or alloy is deposited electrochemically in the primary structures produced by lithography and development. The shape-providing plastic, for example crosslinked PMMA, is subsequently removed from the metallic microstructure in a process step known as delamination. Aggressive chemical solvents are often used for delamination of low-solubility shape-providing plastics; however, chemical modification of the metallic microstructures, which typically comprise, for example, copper, nickel, nickel/cobalt alloy or gold, must be avoided. One such aggressive delamination technique requires fluorination of the low-solubility, shape-providing plastics, for example by means of boiling trifluoroacetic acid, and subsequent dissolution in a mixture of acetone and hydrochloric acid. This treatment causes the plastic to swell, damaging or destroying the metallic microstructure.
It is also known to re-irradiate the shape-providing plastic in a floodlight-like manner after the electroforming step, to increase its solubility; to this end, synchrotron radiation, X-ray radiation or laser radiation is necessary. This process is complex. In the case of moderately high radiation output, this process step is time-consuming; however, the metallic microstructure remains unchanged. At high radiation output, for example high-energy synchrotron radiation, the radiation time is shorter but the surface of the metallic microstructure is modified. Consequent reductions in quality may require complex reworking of the microstructure. Although the metallic microstructures can be covered by masks or inert gas during the irradiation, these steps require complex equipment and production techniques.
It is also known to remove the shape-providing plastic at elevated temperatures and/or in a plasma. However, changes in physical properties, such as hardness and modulus of elasticity of the metals or alloys of the microstructure elements generally occur. Total destruction of the microstructure elements takes place in some areas and chemical reactions take place between the metal of the microstructure elements and the polymer or the ambient atmosphere.
With the aid of the metallic microstructure, microstructure elements are cast from plastic in large numbers, for example by injection moulding, reaction injection moulding or thermoforming. During removal of the microstructure elements made of plastic, the plastic may in the event of a fault remain partly stuck in the metallic microstructure. In this case also, the plastic must be eliminated from the metallic microstructure so that the metallic microstructure can re-used. For thermoplastics, in particular amorphous plastics, suitable solvents are known. However, partially crystalline plastics are frequently only soluble at elevated temperatures. Also problematic in this case are thermosets, for which there are usually no known suitable solvents, or the known solvents attack the metallic microstructure.
Due to the difficulties encountered during delamination, thermosets--such as crosslinked PMMA--and epoxy resins, silicone rubber and other plastics are rarely employed for casting. On the other hand, plastics of this type are frequently desired for applicational reasons, since they are distinguished by thermal and chemical resistance.